1. Field of the Invention
The invention relates to an optical signal generator for generating an intensity-modulated optical signal using an electro-absorption optical modulator.
2. Description of the Related Art
As one of optical modulator for modulating the intensity of light, there can be cited an electro-absorption optical modulator (EA modulator) using an electro-absorption effect of a semiconductor. The electro-absorption effect is a phenomenon that the difference in energy levels between the conduction band and the valence band is varied by applying an electric field to a semiconductor having a quantum well structure, and thus, the absorption of the light propagating through the quantum well structure varies.
The EA modulator is configured including a PIN junction with a double heterostructure having an optical waveguide layer provided with a multiple quantum well (MQW) structure sandwiched by a p-type semiconductor and an n-type semiconductor, a p-side electrode, and an n-side electrode for applying a voltage to the PIN junction.
If a voltage is applied to the PIN junction, an electric field is applied to the optical waveguide layer, and therefore, the light having been input to the optical waveguide layer is absorbed to create electron-hole pairs. As a result, the light passing through the optical waveguide layer decreases, and an OFF state of the optical signal is realized. In contrast, if no voltage is applied to the PIN junction, the light passes through the optical waveguide layer without being absorbed or quenched, and therefore, an ON state of the optical signal is realized.
In other words, the EA modulator varies the absorption of the light propagating through the optical waveguide layer in accordance with the voltage applied to the PIN junction to thereby realize the intensity modulation of the light. For example, by inputting a laser beam into the optical waveguide layer of the EA modulator, and then applying a voltage corresponding to a high-frequency modulation signal between the p-side electrode and the n-side electrode of the EA modulator, it is possible to obtain a modulated signal which is the laser beam modulated with the high-frequency signal.